FIELD OF THE INVENTION
The invention relates to a method for temperature calibration of an optical measurement array for measuring a magnetic field, and to an optical measurement array for measuring a magnetic field.
Optical measurement arrays and measuring methods for measuring a magnetic field using the magnetooptical Faraday effect are known. The Faraday effect is understood to mean the rotation of the plane of polarization of linearly polarized light as a function of a magnetic field. The angle of rotation is proportional to the travel integral across the magnetic field along the path traversed by the light, with what is known as Verdet's constant as the proportionality constant. In general, Verdet's constant is dependent on material, temperature and wavelength. In order to measure the magnetic field, a Faraday sensor device formed of an optically transparent material, such as glass, is placed in the magnetic field. The magnetic field causes a rotation of the plane of polarization of linearly polarized light, transmitted through the Faraday sensor device, by a rotary angle that can be evaluated for a measurement signal. One known application of such magnetooptical measuring methods and measurement arrays is to measure electrical currents. To that end, the Faraday sensor device is placed in the vicinity of a current conductor and detects the magnetic field generated in the current conductor by a current. In general, the Faraday sensor device surrounds the current conductor, so that the measured light circulates around the current conductor in a closed path. In that case, the rotary angle is directly proportional in quantity to the amplitude of the current to be measured. The Faraday sensor device may be constructed as a solid glass ring around the current conductor, or it can surround the current conductor with at least one winding, being constructed as a measurement winding including an optical fiber (fiber coil).
Advantages of magnetooptical measurement arrays and measuring methods over conventional inductive current converters are potential separation and insensitivity to electromagnetic interference. Problems are presented, however, by temperature factors and influences of mechanical bending and vibration in the sensor device and the optical transmission paths, especially optical fibers, for transmitting the measurement light.
A magnetooptical measuring system is known from the Journal of Lightwave Technology, Vol. 12, No. 10, Oct. 1994, pp. 1882 to 1890, in which two light signals pass through an optical series circuit including a first optical fiber, a first polarizer, a Faraday sensor device, a second polarizer and a second optical fiber, in opposed directions of circulation. Both light signals, after passing through the optical series circuit, are converted by corresponding photoelectric converters, each into one electrical intensity signal. A fiber coil including a single-mode fiber with low double refraction is provided as the Faraday sensor device. The polarization axes of the two polarizers form a polarizer angle other than zero degrees with one another and preferably it is 45.degree.. Light from a light source is split into two light signals, and both of those light signals are fed into the Faraday fiber coil, each through an optical coupler and an associated transmission optical fiber on opposite ends. An intensity-standardized measurement signal is derived from two electrical intensity signals I1 and I2, which correspond to the light intensities of the two light signals after passing through the series circuit. The intensity-standardized measurement signal corresponds to the quotient (I1-I2)/(I1+I2) of the difference and the sum of the two intensity signals. Intensity losses in the common light path for the two contrary light signals, and in particular vibration-dictated damping in the two optical fibers, can thus be largely compensated for. The Journal of Lightwave Technology, Vol. 12, No. 10, Oct. 1994, pp. 1882 to 1890 does not describe any compensation of temperature influences on the measurement signal. On the contrary, a temperature-insensitive fiber coil is used as the sensor device. However, producing such fiber coils is problematic.
U.S. Pat. No. 5,008,611 discloses a magnetooptical measurement system, in which a measurement light signal passes through a series circuit including a first polarizer, a Faraday element and a second polarizer (analyzer). In order to minimize the temperature influences resulting from the temperature dependency of the linear double refraction in the Faraday element, the angle between the transmission axis of the first polarizer and the intrinsic double refraction axis in the Faraday element is set to 10.3.degree.. In addition, the angle between the transmission axis of the second polarizer and the intrinsic double refraction axis in the Faraday element is set to 55.3.degree.. That setting of the angular values was ascertained empirically and requires previous knowledge of the intrinsic double refraction axis (characteristic direction) of the Faraday element.